(1) Field of the Invention
The present invention relates to a pulse light-receiving circuit wherein light pulse signal is received, photoelectrically converted and shaped into an electric signal.
(2) Description of the Prior Art
A typical example of a prior art optical fiber system linking configuration is shown in FIG. 1 in which a 50M Baud TTL linking arrangement is schematically illustrated. In this system, a pair of apparatus each made up of a transmitting device T and a receiving device R are disposed in a distance 1 apart from one another and input and output of the data is effected through the linking arrangement.
Of these components, the receiving device the present invention. FIG. 2 shows a prior art example one of such receiving device which includes a single preamplifier A.sub.31 and further includes a peak-holding circuit A.sub.32 for holding a positive peak value V.sub.31 of the output from the preamplifier A.sub.31 and a bottom-holding circuit A.sub.33 which is constructed with the polarities thereof inverted relative to those in the peak-holding circuit A.sub.32, so that a peak-to-bottom value of the pulse (or pulse amplitude) can be obtained. The thus obtained peak-to-bottom value is used as a reference value and compared at a comparator A.sub.34 to an output voltage from the preamplifier A.sub.31.
Another prior art example is shown in FIG. 3. In this figure, designated at A.sub.51 is a preamplifier, which is connected to a noninverting input terminal of a comparator A.sub.53. Here, reference numeral A.sub.52 designates an amplifier with a negative feed back connection made through a diode D.sub.52. A cathode of the diode D.sub.52 is grounded by way of a capacitor C. The diode D.sub.52 is connected by way of a resistor R.sub.52 to an inverting input terminal of a comparator A.sub.53. In this arrangement, the comparator A.sub.53 compares the output value from the preamplifier A.sub.51 with a half of the peak value outputted from the same preamplifier A.sub.51.
In the conventional example previously shown in FIG. 2, if an inphase noise Ns such as power supply noises arises, the peak value of the noise Ns may be held. This would cause the reference value for the comparator A.sub.34 to be shifted as is shown in FIG. 4C, giving rise to a problem that comparator A.sub.34 will not respond to a real input signal to which a response should be made.
Further, the peak hold circuit A.sub.32 differs from the bottom hold circuit A.sub.33 in that these circuits are not perfectly symmetrical. Therefore, the conventional system suffers from a problem that if the system is affected by power supply noise, the system cannot completely compensate for the influence of that noise.
Moreover, in the circuit of the prior art example shown in FIG. 3, half of the peak value of the output from the preamplifier A.sub.51 is compared with the output of A.sub.51. Therefore, if an input pulse light having a duty ratio of 50% has an increased frequency as shown in FIGS. 5A to 5C, the response of the output V.sub.51 from the preamplifier A.sub.51 does not follow the input pulse exactly, as shown in FIG. 5B.
In this case, the bottom value of the output from the preamplifier A.sub.51 is not reduced to a non-signal level, and the potential level of the comparative reference voltage V.sub.53 for the comparator A.sub.53 does not agree with a potential level corresponding to a median amplitude of the output of A.sub.51. Therefore, the waveform of the output V.sub.54 from the comparator A.sub.53 is distorted from a duty ratio of 50%.
On the other hand, Japanese Patent Application Laid-Open Sho 58 No.114637 discloses a publicly known prior art example of light pulse receivers. Peak and minimum values of a pulse signal are obtained by a peak value-holding circuit and a minimum value-holding circuit, respectively so as to yield an amplitude of the pulse signal from the difference. Then, a half of the pulse amplitude is used as a reference value for a comparator in order to compare the original pulse signal therewith. In this configuration, however, the peak value-holding circuit and the minimum value-holding circuit are different circuits. As a result, in the presence of noise such as power supply noise, these circuits hold a peak value and a minimum value of the noise. This method therefore suffers from a drawback that the reference value used in the comparator erroneously set by that noise.